Process for producing carbon fibers

ABSTRACT

Carbon fibers are produced by carbonizing a partially carbonized cellulosic base fiber by subjecting it to a temperature within the range of from 1,900* to 2,100* C. while under an applied tensional force. The carbon fibers produced in this manner are capable of withstanding the application of high stress during subsequent graphitization without breaking and can be stretched to a high degree.

United States Patent I Sloka [4 1 Mar. 28, 1972 [s41 PROCESS FOR PRODUCING CARBON 3,454,362 7/1969 Spry ..23/2o9.1 FIBERS 3,529,934 9/1970 Shindo ..23/209.l [72] Inventor: William Edward Sloka, Fostoria, Ohio FOREIGN PATENTS OR APPLICATIONS Assignee= Union Carbide Corporation, New York, 1,148,874 4/1969 Great Britain ..23/209.1

[22] Filed: buy 30, 1969 Primary Examiner-Edward J. Meros Attorney-Paul A. Rose, Robert C. Cummings and John S. [21] Appl. No.: 846,228 Pis itello 52 U.S. c1 ..23/209.4, 23/2091, 23/2092 [571 ABSTRACT [51] Int.Cl. ..C0lb 31/07 Carbon fibers are produced b cal-honking a partially can Fldd of Search 1 21209.4; bonized cellulosic base fiber subjecting it to a temperature 264/29 8/116 within the range of from 1,900 to 2,l00 C. while under an applied tensional force. The carbon fibers produced in this [56] Rdennm Cited manner are capable of withstanding the application of high UNITED STATES PATENTS stress during subsequent graphitization without breaking and can be stretched to a high degree. 3,305,315 2/1967 Bacon et al. ..23/209.1 3,412,062 1 1/1968 Johnson et al. ..260/37 12 Claims, No Drawings PROCESS FOR PRODUCING CARBON FIBERS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved process for producing carbon fibers from cellulosic materials and to the fibers so produced. As used herein and in the appended claims, carbon is intended to include both the non-graphitic and graphitic forms of carbon.

2. Description of the Prior Art Carbon is an element which possesses many interesting arid useful chemical and physical properties. It is a material which both can be found in nature and produced synthetically.-Carbon is readily processible material and can be fashioned into almost any intricate shape or pattern. Today, the uses of carbon in commerce and industry are myriad.

Presently, most of the carbon articles used in industry are produced by a process which comprises mixing non-graphitic carbon particles with a carbonizable binder, extruding or molding the so-produced mixture into the desired shape or article and, subsequently, heating it to a temperature sufficient to carbonize the binder phase. If, during this heating the maximum temperature which the resultant article experiences is of the order of 700900 C., it is said to be a non-graphitic all carbon article. However, if the article is further heated to a temperature of the order of 2,0002,500 C. and higher, it is said to be converted to a graphitic form of carbon and is generally called graphite.

Recently, there has been introduced to the carbon art carbon in the form of a textile. This form of carbon is unique in that it possesses the flexibility of a textile while at the same time is characterized by the electrical and chemical properties associated with conventionally formed carbon articles.

U.S. Pat. No. 3,01 1,981 which issued Dec. 5, 1961 to W. T. Soltes describes and claims a method for manufacturing carbon in a textile form. Briefly, the process disclosed therein comprises heating a cellulosic textile in an inert atmosphere at a progressively higher temperature until substantial carbonization of the textile occurs. The resultant product possesses the chemical and physical attributes exhibited by conventionally formed carbon articles while at the same time it retains the flexibility and other physical characteristics associated with the textile starting material, such as hand and drape.

A textile form of fibrous graphite is disclosed and claimed in U.S. Pat. No. 3,l07,l52, which issued to C. E. Ford and C. V. Mitchell on Oct. l5, 1963. Broadly stated, the process for producing fibrous graphite disclosed therein comprises heating a cellulosic starting material in an inert atmosphere at progressively higher temperatures for various times until a temperature of about 900 C. is achieved followed by further heating in a suitable protective atmosphere at higher temperatures until substantial graphitization occurs. The product produced by this process exhibits the chemical and physical properties generally associated with conventionally fabricated graphite while, at the same time, it retains the textile characteristics of the starting material.

Recently, a high modulus, high strength form of graphite fiber has become commercially available. Briefly, this material is produced by a process which comprises stretching a substantially all carbon fiber while it is being heated to graphitizing temperatures. Although this improved form of graphite fiber possesses properties which are unobtained in graphite fibers produced via the methods disclosed by both Soltes and Ford et al., the method of producing it suffers from at least one serious processing difficulty. Namely, the high force necessary to achieve both maximum strength and a high Young's modulus is a limiting factor during the stress graphitization of the already carbonized fiber. That is, in order to obtain optimum strength and modulus values, the amount of stress required is dangerously close to the breaking stress of the carbon fiber. Needless to say, such close limits are not conducive to a successful commercial operation.

In application, Ser. No. 610,789, filed Jan. 23, 1967, now abandoned, a process for producing non-graphitic carbon fibers which are especially amenable to conventional stress graphitizing treatments is described. Briefly, that process comprises concurrently longitudinally stressing a partially carbonized cellulosic base fiber while subjecting it to a carbonizing temperature in the range of from about 250 to 900 C. so that a given length of the resultant, stretched fiber is at least 5 percent longer than it would have been had it been carbonized in a stress free manner. The so-produced non-graphitic carbon fibers exhibit a higher Young's modulus of elasticity than previously obtainable in non-graphitic carbon fibers produced by conventional techniques. According to that application, when non-graphitic carbon fibers produced in this manner were subsequently stress graphitized at a force of 400 grams per two ply they exhibited a Young's modulus and breaking strength of 52 X 10 lb./in and 280,000 lb./in, respectively, while fibers produced by simply stress graphitizing a conventionally carbonized fiber required a force of 1300 grams per 2 ply to duplicate these physical properties.

SUMMARY OF THEINVENTION In accordance with the instant invention, it has now been discovered that carbon fibers which are stronger and even more amenable to conventional stress graphitizing treatments than carbon fibers produced in accordance with application, Ser. No. 610,789 can be produced by carbonizing a partially carbonized cellulosic base fiber by subjecting it to a carbonizing temperature within the range of from l,900 to 2,l00 C., preferably from l,950 to about 2,050 C., while under an applied tensional force so that the resultant stretched fiber is at least 5 percent longer than it would have been had it been carbonized in a stress-free manner. The carbon fibers produced in this manner are capable of withstanding the application of significantly higher stress during subsequent graphitization without breaking than fibers produced in accordance with application, Ser. No. 6l0,789. As a result these fibers can be stretched to a greater degree than the fibers produced in accordance with application, Ser. No. 6l0,789. As is well known, the more that carbonized yarn is stretched during graphitization the higher are the tensile strength and Youngs modulus of the filaments of the resulting yarn.

DESCRIPTION OF THE PREFERRED EMBODIMENT Fibers suitable for the practice of the invention are those which upon carbonization do not melt of fuse but which when so heat treated tend to lose their inherent orientation. Specifically, fibers suitable for the practice of the invention are fibers of either natural or regenerated cellulosic origin which have been subjected to a pre-heat treatment to convert them to partially carbonized carbonaceous fibers. This is accomplished by first heating the raw cellulosic base fibers in either an inert or oxidizing atmosphere to a temperature in the range of from about to about 350 C. for fibers which have been treated with a carbonizing aid, such as phosphoric acid, or from about to about 350 C. for fibers which are untreated. Both of these techniques are described in detail in U.S. Pat. No. 3,305,315 which has been assigned to the same assignee as the instant application.

The present invention will now be described in greater detail in the following example.

EXAMPLE I An apparatus was constructed for stretching carbonaceous fibers, preferably in yarn form, at elevated temperatures. This apparatus consisted of two sets of canted stainless steel rolls, one for yarn payoff and the other for yarn takeup, mounted at opposite ends of a hollow electric resistance heated tube furnace about 18 inches in length. The drive motors for the rolls were connected to a control unit, and the rolls could be run at any desired ratio of takeup-to-payoff speeds, thereby controlling the actual shrinkage permitted to the yarn or the actual stretch applied to the yarn during its passage through the furnace. A yarn tension monitoring device with recorder was mounted between the yarn payoff rolls and the furnace. A nitrogen atmosphere was maintained within the electric furnace during operation to protect the yarn against damage by oxidation. The furnace temperature was read with an optical pyrometer.

The furnace was heated to a temperature-of 1,350" C. and a partially pre-carbonized yarn (prepared by heating a 2 ply, 720 filaments per ply, 1,650 denier rayon yarn to a temperature of about 250 C.) was passed through the furnace while the yarn takeup and payoff rolls were operated so as to put the yarn under an applied tension or stress. The stress carbonized yarn prepared in this manner was then cooled to room temperature and subsequently stress-graphitized under the maximum tension that the yarn could withstand without frequent breakage (predetermined by applying varying tension to the yarn). A furnace similar to that employed for stress carbonizing the yarn was employed for the stress graphitization. The furnace was heated to 2,900 C. for this purpose.

The procedure was repeated employing successively higher stress-carbonization temperatures. The strength of the resulting yarn was found to increase as the stress-carbonization temperature was increased. Table I below presents data illustrating the Young's modulus and tensile strength of the individual filaments of yarn stress carbonized at varying temperatures by the foregoing technique and subsequently stress graphitized at 2,900 C.

it should be noted that the partially carbonized cellulosic starting material inherently shrinks while it is being completely carbonized. The change in the length of any given length of partially carbonized yarn due to its passage through the carbonization furnace is easily computed from the difference in speeds of the takeup and the payoff rolls. The percent of effective stretch can then be determined by taking the difference in length between a unit length of stress-carbonized material and a similar unit length of material carbonized in a stress-free manner and dividing that value by the length of the stress-free carbonized material followed by multiplying the obtained value by 100. The foregoing is the meaning to be applied to the term percent effective stretch" as used herein and in the appended claims.

iapsoi-l-eiii east ar ee s .I h V.

l. A process for producing non-graphitic carbon fiber which comprises carbonizing a partially carbonized carbonaceous fiber produced by the heat treatment of a fiber of cellulosic origin at a temperature in the range of from about 100 to about 350 C. by subjecting it to an initial carbonizing temperature within the range of from l,900 to 2,l00 C. while under an applied tensional force sufficient to cause a percent efi'ective stretch of at least 5 percent of the fiber.

2. The process of claim 1 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin at a temperature in the range of from about 150 to about 350 C. y

3. The process of claim 1 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin which has been treated with a carbonizing aid to a temperature in the range of from about 100 to about 350 C.

4. The process of claim 3 wherein said carbonizing aid is phosphoric acid.

5. The process of claim 1 wherein said non-graphitic carbon fiber is stress graphitized by heating said fiber to a temperature of about 2,900 C. while applying a stressing force thereto sufficient to permanently stretch said fiber.

6. The process of claim 1 wherein said partially carbonized carbonaceous fiber is in yarn form.

7. The process of claim 1 wherein said partially carbonized carbonaceous fiber is carbonized at a temperature of from l,950 C. to 2,050 C.

8. The process of claim 7 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin at a temperature in the range of from about 150 to about 350 C.

9. The process of claim 7 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin which has been treated with a carbonizing aid to a temperature in the range of from about 100 to. about 350 C.

10. The process of claim 9 wherein said carbonizing aid is phosphoric acid.

11. The process of claim 1 wherein said partially carbonized carbonaceous fiber is carbonized at a temperature of from g 1 Properties measured on filaments 2 cm. in length, averages of 6 tests per sample.

As is apparent from Table 1, those yarns capable of withstanding the greatest applied tensile force and, therefore, capable of being stretched the most during a stress-graphitization, exhibited the highest tensile strength and Young's modulus. The samples exhibiting the highest tensile strength and Young's modulus were those which had been stress-carbonized i h a e, mesretutak wmu 593% l,950 to 2,050 C. and said non-graphitic carbon fiber is stress graphitized by heating said fiber to a temperature of about 2,900 C..while applying a stressing force thereto sufficient to permanently stretch said fiber.

l2. The process of claim 7 wherein said carbonaceous fiber is in yarn form.

partially carbonized "was mm Manama" CERTIFICATE OF CORRECTION mm no. 1 Date March 28, 1972 k William Edward S loka It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- Column 1, line 14, insert f r "readily".

Column 2, lines 1, 27 and 41, delete the comma each occurrence, after "application"; line 39, delete the comma(,) afte "plication". v "11 p.s.i." should read "10 p.s.i."; lin

Column 3, Table" I, 64, delete "a"'before "stress-graphitiza-".

Signed and sealed this 27th day of March 1973.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR. Attesting Officer Commissioner of Patents Pat 212= +.72 

2. The process of claim 1 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin at a temperature in the range of from about 150* to about 350* C.
 3. The process of claim 1 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin which has been treated with a carbonizing aid to a temperature in the range of from about 100* to about 350* C.
 4. The process of claim 3 wherein said carbonizing aid is phosphoric acid.
 5. The process of claim 1 wherein said non-graphitic carbon fiber is stress graphitized by heating said fiber to a temperature of about 2,900* C. while applying a stressing force thereto sufficient to permanently stretch said fiber.
 6. The process of claim 1 wherein said partially carbonized carbonaceous fiber is in yarn form.
 7. The process of claim 1 wherein said partially carbonized carbonaceous fiber is carbonized at a temperature of from 1,950* to 2,050* C.
 8. The process of claim 7 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin at a temperature in the range of from about 150* to about 350* C.
 9. The process of claim 7 wherein said partially carbonized carbonaceous fiber is produced by heating a fiber of cellulosic origin which has been treated with a carbonizing aid to a temperature in the range of from about 100* to about 350* C.
 10. The process of claim 9 wherein said carbonizing aid is phosphoric acid.
 11. The process of claim 1 wherein said partially carbonized carbonaceous fiber is carbonized at a temperature of from 1,950* to 2,050* C. and said non-graphitic carbon fiber is stress graphitized by heating said fiber to a temperature of about 2, 900* C. while applying a stressing force thereto sufficient to permanently stretch said fiber.
 12. The process of claim 7 wherein said partially carbonized carbonaceous fiber is in yarn form. 